The invention relates generally to methods for directing migration of non-aqueous phase liquids (NAPLs) from NAPL-contaminated sediment, and relates more particularly to methods of directing NAPL migration from such sediment to the surface of a body of water using devices through which NAPLs and other contaminants cannot percolate. The invention also contemplates directing migration of other granular materials that may contain contaminants, such as sand and silt, in a fluid, such as in a gas (i.e., via advective flow) or in a liquid (i.e., via conductive flow), especially where ebullition may be a factor.
NAPLs are highly hydrophobic liquids that have a low solubility and a low surface tension. Consequently, they are not water-soluble and form a separate phase when mixed with water. For example, oil is a NAPL that does not mix with water, and oil and water in a glass will separate into two separate phases. NAPLs can be lighter than water (light NAPLs (LNAPLs)) or denser than water (dense NAPLs (DNAPLs)). Hydrocarbons, such as gasoline, oil creosote and tar, as well as chlorinated solvents, such as trichloroethylene, are examples of NAPLs.
NAPLs are often found at or near industrial sites or former industrial sites. For example, historic wastewater outfalls from manufactured gas plant (MGP) sites often contain NAPL-contaminated sediment, such as MGP tar (e.g., coal tar), as well as other organic matter. Likewise, current or historical wastewater discharges from other industrial sites, such as refineries, wood-treating facilities, asphalt plants, aluminum smelters, coking plants, steel mills, chemical manufacturing facilities and others, may also contain NAPLs (both DNAPLs and LNAPLs).
When organic matter in NAPL-contaminated sediment anaerobically biodegrades, carbon dioxide (CO2) and methane (CH4) gases are produced that can migrate toward the surface as buoyant bubbles, entraining and transporting the highly hydrophobic NAPLs upward through a water column to the water surface, even though NAPL by itself may be denser than water. When the gas is released to the atmosphere at the water surface, NAPLs typically form a surface sheen because the net density of the gas bubble/NAPL is less than water and/or because the surface tension of NAPLs is much less than that of water. The sheen and NAPL droplets then are able to migrate along the water surface, creating a potential human health hazard and environmental hazard (e.g., coal tar is a known human carcinogen). Likewise, NAPL droplets adhere to shoreline structures or floating objects, such as boats or buoys. Occasionally, NAPL droplets lose their buoyancy when the gas is lost or when the surface tension of the water is broken. The NAPL droplets then fall back through the water column only to be re-deposited on the sediment. NAPL migration can also occur when ebullition or some other force (e.g., turbulent prop wash) disturbs sediment and causes NAPLs to migrate from the sediment. LNAPL from the sediment may also move upward due to buoyancy.
Certain techniques for directing NAPL migration are known to one of ordinary skill in the art and are summarized by McAnulty & McLinn. See McAnulty S & McLinn E, “NAPL migration from contaminated sediment 2: implications for remedial design,” Paper A-031 in Remediation of Contaminated Sediments—2007 (Foote E & Durell, eds. 2007); see also McLinn E & McAnulty S, “NAPL migration from contaminated sediment 1: diagnosis and transport mechanisms,” Paper A-030 in Remediation of Contaminated Sediments—2007 (Foote E & Durrell, eds. 2007), each of which is incorporated herein by reference as if set forth in its entirety. These techniques include, but are not limited to, the following: (1) removing NAPL-contaminated sediment; (2) placing filter caps over NAPL-contaminated sediment; (3) infilling over NAPL-contaminated sediment; (4) lowering the temperature of NAPL-contaminated sediment; (5) increasing the pressure over NAPL-contaminated sediment; (6) altering the properties of NAPL-contaminated sediment; and (7) preventing physical disturbance of NAPL-contaminated sediment. Each technique, however, presents its own advantages and disadvantages. The art, however, needs other devices and methods of controlling or preventing NAPL migration from NAPL-contaminated sediments.